The field of the invention relates generally to variable reluctance speed sensors, and more specifically, to signal conditioning of variable reluctance speed sensor output signals.
At least some known variable reluctance sensors include wire wrapped around a permanent magnet and are configured to sense a steel target that is part of a rotating assembly. For example, in an industrial application, the steel target may be coupled to, and rotate with, a rotor of a generator. As the steel target passes the variable reluctance sensor, a magnetic field produced by the sensor is disrupted, producing a sinusoidal voltage signal. Variable reluctance speed sensors are self-powered devices that provide an approximately 0.7 Volt peak-to-peak output at relatively low engine speeds and an approximately 50 Volt peak-to-peak output at relatively high speeds. The low output voltage signals generated at low engine speeds are difficult to accurately measure by the engine control circuitry and often require extensive filtering and signal amplification stages. The frequency and amplitude of the sinusoidal voltage signal are proportional to the speed of the rotor. The amplitude of the sinusoidal voltage signal output by the sensor is also dependent upon the distance between the sensor and the steel target. Because variable reluctance sensors do not require external power to generate an output signal, they are also referred to as “passive” magnetic sensors. The sensor output is provided to remote signal-processing circuitry for analysis. The signal-processing circuitry is positioned remotely from the sensor due to high temperatures and/or other harsh environmental conditions present at the position of the sensor. Due in part to the remote positioning of the signal-processing circuitry, if the circuitry is configured to analyze sensor output from a high speed rotor, the sensor may not produce an output signal with an adequate amplitude for signal-processing when the rotor is rotating at a low speed. However, incorporating signal conditioning circuitry within the speed sensor is problematic for two reasons, the high temperature environment (180° C.+) does not permit the use of cost effective integrated circuits, and employing electronics within sensors typically increases the number of wires and weight required to implement the interconnecting speed sensor and engine control harness.